Network node and method for managing transmission of cell reference symbols

ABSTRACT

A method performed by a network node for managing transmission of Cell Reference Symbols, CRS, wherein the network node  110  operates one or more cells and the network node  110  is configured to transmit the CRS in a first bandwidth mode. When the network node  110  has identified a cell  130  which is not actively serving any UEs  120 , also referred to as an empty cell, the network node  110  applies ( 302 ) a reduced CRS bandwidth mode in the first cell  130  in relation to the first bandwidth mode. By applying ( 302 ) a reduced CRS bandwidth mode in the empty cell  130 , the overall interference of the CRS from the empty cell  130  is reduced, thereby enhancing the performance in cells actively serving UEs  120.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of application Ser. No. 15/506,474,having a section 371(c) date of Feb. 24, 2017 (published as US20170288822 on Oct. 5, 2017), which is a 35 U.S.C. § 371 National Stageof International Application No. PCT/SE2014/050994, filed Aug. 28, 2014.The above identified applications and publication are incorporated bythis reference.

TECHNICAL FIELD

Embodiments herein relate to a network node and a method therein. Inparticular, it relates to a method for managing transmission of CellReference Symbols.

BACKGROUND

Communication devices such as User Equipments (UEs) are enabled tocommunicate wirelessly in a cellular communications network or wirelesscommunication system, sometimes also referred to as a cellular radiosystem or cellular networks. The communication may be performed e.g.between two UEs, between a UE and a regular telephone and/or between aUE and a server via a Radio Access Network (RAN) and possibly one ormore core networks, comprised within the cellular communicationsnetwork.

UEs may further be referred to as wireless terminals, mobile terminalsand/or mobile stations, mobile telephones, cellular telephones, laptops,tablet computers or surf plates with wireless capability, just tomention some further examples. The UEs in the present context may be,for example, portable, pocket-storable, hand-held, computer-comprised,or vehicle-mounted mobile devices, enabled to communicate voice and/ordata, via the RAN, with another entity, such as another wirelessterminal or a server.

The cellular communications network covers a geographical area which isdivided into cell areas, wherein each cell area being served by anetwork node. A cell is the geographical area where radio coverage isprovided by the network node.

The network node may further control several transmission points, e.g.having Radio Units (RRUs). A cell can thus comprise one or more networknodes each controlling one or more transmission/reception points. Atransmission point, also referred to as a transmission/reception point,is an entity that transmits and/or receives radio signals. The entityhas a position in space, e.g. an antenna. A network node is an entitythat controls one or more transmission points. The network node may e.g.be a base station such as a Radio Base Station (RBS), eNB, eNodeB,NodeB, B node, or BTS (Base Transceiver Station), depending on thetechnology and terminology used. The base stations may be of differentclasses such as e.g. macro eNodeB, home eNodeB or pico base station,based on transmission power and thereby also cell size.

Further, each network node may support one or several communicationtechnologies. The network nodes communicate over the air interfaceoperating on radio frequencies with the UEs within range of the networknode. In the context of this disclosure, the expression Downlink (DL) isused for the transmission path from the base station to the mobilestation. The expression Uplink (UL) is used for the transmission path inthe opposite direction i.e. from the UE to the base station.

In 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE),base stations, which may be referred to as eNodeBs or even eNBs, may bedirectly connected to one or more core networks. In LTE the cellularcommunication network is also referred to as E-UTRAN.

An E-UTRAN cell is defined by certain signals which are broadcasted fromthe eNB. These signals contain information about the cell which can beused by UEs in order to connect to the network through the cell. Thesignals comprise reference and synchronization signals which the UE usesto find frame timing and physical cell identification as well as systeminformation which comprises parameters relevant for the whole cell.

A UE needing to connect to the network must thus first detect a suitablecell, as defined in 3GPP TS 36.304 v11.5.0. This is performed bymeasuring on received reference signals sent by neighboring cells, alsoreferred to as “listening” for a suitable cell. The suitable cell iscommonly the cell with best quality of signal. Listening for a suitablecell may comprise searching for reference signals transmitted from thenetwork node in an OFDM subframe. When a suitable cell is found the UEperforms random access, according to a system information for the cell.This is done in order to transmit a Radio Resource Control (RRC)connection setup request to the network node. Assuming the random accessprocedure succeeds and the network node receives the request, thenetwork node will either answer with an RRC connection setup message,which acknowledges the UEs request and tells it to move into RRCconnected state, or an RRC connection reject, which tells the UE that itmay not connect to the cell. In RRC connected state the parametersnecessary for communication between the network node and the UE areknown to both entities and a data transfer between the two entities isenabled.

To facilitate handover to other cells, each network node may store cellidentities that are supported by the other network nodes in an addressdatabase, in order to know how to contact the network node of potentialtarget cells for handover. Each network node serving a cell typicallystores in the data base which cells it has neighbor relations to, i.e.which of the cells in the area UEs often perform handover to. The cell'sneighbor relations will hereafter be referred to as the cell's “neighborrelation list”.

Cell specific Reference Signals (CRS) are UE known symbols that areinserted in a Resource Element (RE) of a subframe of an OrthogonalFrequency-Division Multiplexing (OFDM) time and frequency grid andbroadcasted by the network node. Each RE has an extension in thefrequency domain corresponding to an OFDM sub carrier and an extensionin the time-domain corresponding to an OFDM symbol interval.

The CRS are used by the UE for downlink channel estimation. Channelestimation is used for demodulation of downlink data both when the UE isin RRC connected state and is receiving user data and when the UE is inRRC idle state and is reading system information. Due to the latter usecase, the CRSs must be transmitted even from cells which do not have anyUEs in RRC connected state since the eNB cannot know whether a UE wantsto access the network until it performs random access. Downlink cellspecific reference signals are inserted within the first and third lastOFDM symbol of each slot with a frequency domain spacing of sixsub-carriers. A slot is a time period of the OFDM time and frequencygrid, which is usually 0.5 msec long. A problem with the knowntechnology is therefore that cells without any UEs in RRC connectedstate still consume power due to CRS broadcasting.

In case several antennas are used by the network node for transmittingand each antenna is representing a cell, each antenna has to transmit aunique reference signal in order for the UE to connect to that specificcell. When one antenna transmits, the other antennas have to be silentin order not to interfere with the first antennas reference signal. Toreduce the interference of reference signals between the cells, theposition of the CRS is usually shifted in frequency between the cells.The CRS can be shifted between 0-5 sub carriers, each sub carriercorresponding to a frequency shift of 15 kHz for LTE. The cell specificfrequency shift can be derived from the physical Cell Identity (Cell ID)which is signaled to the UE by selection of appropriate PrimarySynchronization Channel (PSCH) and Secondary Synchronization Channel(SSCH).

Although this reduces the interference of reference symbols betweencells, it has the problem that the reference symbols of one cell willdisturb Physical Downlink Shared Channel (PDSCH) and Physical DownlinkControl Channel (PDCCH) symbols of neighboring cells.

Hence, even though cells do not have any UEs in RRC connected state,disturbance may impact UE DL throughput in neighboring cells. This willespecially be the case when the UE is in and/or close to borders betweencells.

Reducing the power of the CRS may mitigate this problem. However, inorder to access a cell the UE must be able to hear the CRS of the cell,i.e. the UE must be able to recognize and receive the CRS transmittedfrom the cell. Therefore reducing the power of the CRS also shrinks thesize of the cell, since more distant UEs no longer will hear the CRS.Furthermore, the quality of the channel estimates used for demodulationdecreases when the Signal to Interference Ratio (SINR) on the CRSdecreases. Reducing the power of the CRS therefore causes degradation ofcell edge performance. This degradation is further aggravated when theload in the network increases, especially if the data is transmittedwith higher power than the CRS, which is usually the case when theeffect of CRS interference is to be reduced.

SUMMARY

It is therefore an object of embodiments herein to enhance theperformance in a wireless communications network.

According to a first aspect of embodiments herein, the object isachieved by a method performed by a network node for managingtransmission of Cell Reference Symbols, CRS. The network node operatesone or more cells and is configured to transmit the CRS in a firstbandwidth mode. When a first cell is identified, which first cell is notactively serving any UEs, the network node applies a reduced CRSbandwidth mode on CRS which are transmitted in the first cell. In thereduced bandwidth mode the bandwidth is reduced in relation to the firstbandwidth mode.

According to a second aspect of embodiments herein, the object isachieved by a network node for performing the method for managingtransmission of Cell Reference Symbols, CRS. The network node operatesat least one cell and is configured to transmit the CRS in a firstbandwidth mode. The network node is configured to identify a first cell,which first cell is not actively serving any UEs. The network nodefurther is configured to apply a reduced CRS bandwidth mode on CRS whichare transmitted in the first cell, in relation to the first bandwidthmode.

By applying a reduced bandwidth mode on CRS in cells that do not serveany UEs in RRC connected mode, the power consumption and theinterference from empty cells can be reduced, thereby enhancing theperformance of cells that have UEs in RRC connected mode.

BRIEF DESCRIPTION OF THE DRAWINGS

Examples of embodiments herein are described in more detail withreference to attached drawings in which:

FIG. 1 is a schematic block diagram illustrating embodiments of awireless communications network.

FIG. 2 is a schematic block diagram illustrating embodiments of an OFDMsubframe.

FIG. 3 is a flowchart depicting embodiments of a method in a networknode.

FIG. 4 is a schematic block diagram illustrating embodiments of anetwork node.

DETAILED DESCRIPTION

FIG. 1 depicts an example of a wireless communications network 100according to a first scenario in which embodiments herein may beimplemented. The wireless communications network 100 is a wirelesscommunication network such as an LTE, E-Utran, WCDMA, GSM network, any3GPP cellular network, Wimax, or any cellular network or system.

The wireless communications network 100 comprises a plurality of networknodes whereof two, a first network node 110 and a second network node111 are depicted in FIG. 1. The first network node 110 and the secondnetwork node 111 are network nodes which each may be a transmissionpoint such as a radio base station, for example an eNB, an eNodeB, or anHome Node B, an Home eNode B or any other network node capable to servea wireless terminal such as a user equipment or a machine typecommunication device in a wireless communications network. The firstnetwork node 110 and the second network node 111 each serves a pluralityof cells 130, 131, 132.

The wireless communications network 100 comprises a UE 120. The firstnetwork node 110 and the second network node 111 may each be atransmission point for the wireless terminal 120. The UE 120 is withinradio range of the first network node 110 and the second network node111, this means that it can hear signals from the first network node 110and the second network node 111.

The UE 120 may e.g. be a wireless terminal, a wireless device, a mobilewireless terminal or a wireless terminal, a mobile phone, a computersuch as e.g. a laptop, a Personal Digital Assistant (PDA) or a tabletcomputer, sometimes referred to as a surf plate, with wirelesscapability, or any other radio network units capable to communicate overa radio link in a wireless communications network. Please note the termwireless terminal used in this document also covers other wirelessdevices such as Machine to machine (M2M) devices.

FIG. 2 shows an exemplary downlink OFDM time and frequency grid, whichis also referred to as an OFDM subframe. Each subframe comprises twoslots. Each slot comprising a number of resource elements (RE) 201extending both in the time domain (x-axis) and in the frequency domain(z-axis). Each RE's 201 extension in the frequency domain is referred toas a sub-carrier whereas the extension in the time domain is referred toas an OFDM symbol. In the time domain, LTE downlink transmissions areorganized into radio frames of 10 ms, wherein each radio frame comprisesten equally-sized subframes. Furthermore, the resource allocation in LTEis typically described in terms of Physical Resource Blocks (PRBs),comprising a plurality of REs. A resource block corresponds to one slotin the time domain and 12 contiguous subcarriers in the frequencydomain.

Downlink and uplink transmissions are dynamically scheduled, i.e. ineach subframe the network node 130 transmits control information aboutto or from which UE 120 data is transmitted and upon which resourceblocks the data is transmitted. The control information may comprisesystem information, paging messages and/or random access responsemessages. The control information for a given UE 120 is transmittedusing one or multiple Physical Downlink Control Channels (PDCCH).Control information of a PDCCH is transmitted in the control region ofeach subframe. FIG. 2 shows an exemplary size of a normal control regionof three OFDM symbols allocated for control signaling, for example thePDCCH. The size of the control region may however be dynamicallyadjusted according to the current traffic situation. In the exampleshown in the figure only the first OFDM symbol out of the three possibleis used for control signalling. Typically the control region maycomprise many PDCCHs carrying control information to multiple UEs 120simultaneously. The REs used for control signaling are indicated withwave-formed lines and REs used for CRS are indicated with diagonallines.

The CRS are used by the UE 120 for downlink channel estimation. Channelestimation is used for determining the demodulation of downlink databoth when the UE 120 is in RRC connected state and is receiving userdata and when the UE 120 is in RRC idle state and is reading systeminformation. Downlink CRS are inserted within the first and third lastOFDM symbol of each slot with a frequency domain spacing of sixsub-carriers.

The subframe also comprises data symbols used for transmitting user databetween the network node 110 and the UE 120. The data symbols aresituated in the region following the control region, which is alsoreferred to as the data region.

Example of embodiments of a method in the network node 110 for managingtransmission of Cell Reference Symbols, CRS, will now be described withreference to a flowchart depicted in FIG. 3. The network node 110operates one or more cells and is configured to transmit the CRS in afirst bandwidth mode during operation. This relates to normal operation.The first bandwidth mode may also be referred to as the normal bandwidthmode which is used when the at least one cell of the network node 110 isserving at least one UE 120 in RRC connected mode. In the normalbandwidth mode CRS are transmitted over the entire available bandwidthof a DL Radio Frame (RF), i.e. CRS are transmitted in all PhysicalResource Blocks (PRBs) of the cell.

The method comprises the following actions, which actions may be takenin any suitable order. Dashed lines of a box in FIG. 3 indicate thatthis action is not mandatory.

Action 301

The network node 110 identifies a first cell 130 which is not activelyserving any UEs 120. When the cell is not actively serving any UEs 120,the cell is referred to as an empty cell. The cell is not activelyserving any UEs 120 when the network node 110 has not sent or receivedany messages to/from any UEs 120 in the cell within a predeterminedtime, and/or when the cell does not have any UEs 120 in RRC connectedmode.

The cell may switch from not actively serving any UEs 120 to activelyserving UEs in case of certain events. Events that trigger a switch maye.g. be that the network node 110 sends a page message in the cell 130,receives a random access preamble in the cell 130 or sends a randomaccess response in the cell 130. It may further be triggered when thenetwork node sends/receives Common Control Channel messages, DedicatedControl Channel messages and/or Dedicated Traffic Channel messages inthe cell 130.

Action 302

When the network node 110 has identified a first cell 130 which is notactively serving any UEs 120, i.e. an empty cell 130, the network node110 applies a reduced bandwidth mode in the first cell 130. In thereduced bandwidth mode the bandwidth is reduced in relation to the firstbandwidth mode. This reduced bandwidth mode may also be referred to aslow bandwidth mode. Low bandwidth mode means that the network node 110is not transmitting CRS in all PRBs of the cell. By reducing thebandwidth of the CRS, i.e. only transmitting CRS on a part of theavailable bandwidth of the DL RF, the overall interference of the CRSfrom the empty cell 130 is reduced. Reducing the interference from theempty cell 130 increases the throughput in cells 131, 132 with RRCconnected UEs 120 is increased.

In one embodiment herein, the reduced CRS bandwidth mode is applied onCRS which are sent in any subframe, except in subframes where thenetwork node 110 transmits system information, paging or random accessresponse messages or subframes where the network node 110 assumes a UE120 to perform measurements. This is usually the case in the firstsubframe of the DL RF, however this information may also be sent inother subframes. By applying the reduced CRS bandwidth mode in allsubframes except the ones mentioned above, the interference by the CRSis reduced while at the same time allowing UEs 120 in neighboring cells131, 132 to hear the CRS from the empty cell 130. This is required inorder for the UE 120 to get information about the modulation of thesignal, in order to be able to demodulate the downlink control channelof the cell. In this embodiment the network node 110 may send CRS overthe entire bandwidth of the subframe in which the network node 110transmits system information, paging or random access response messagesor assumes UE 120 to perform measurements.

In another embodiment herein the network node 110 applies the reducedCRS bandwidth mode on CRS which are sent in any subframe, except in thefirst OFDM symbol of a subframe in which the network node 110 transmitssystem information, paging or random access response messages or assumesUE 120 to perform measurements. In this embodiment the network node 110sends CRS over the entire bandwidth of the first OFDM symbol of thesubframe in which the network node 110 transmits system information,paging or random access response messages or assumes UE 120 to performmeasurements. This has the benefit that the interference from the emptycell 130 is reduced over a large portion of the time domain, while stillallowing the UE 120 to hear the CRS from the empty cell 130 over theentire bandwidth in OFDM symbols where the UE 120 is supposed to listenfor CRS.

In a further embodiment herein the network node 110 sends CRS only inPRBs which are used for transmission of data or control information inthe subframe in which the network node 110 transmits system information,paging or random access response messages or assumes UE to performmeasurements.

The network node 110 may also send CRS only in REs, which are adjacentto REs which are mapped to a common search space of PDCCH. Thereby CRSare sent only in areas where the UE is looking for PDCCH.

The network node 110 holds the reduced CRS bandwidth mode as long as thecell 130 is determined not to actively serve any UEs 120. As soon as thecell is actively serving a UE 120, the network node 130 switches to thenormal bandwidth mode, thereby transmitting CRS over the entirebandwidth of the subframes.

The CRS bandwidth may further be adapted in several levels. For LTE, thebandwidth in the cell 130 may e.g. be varied in levels between 1.4 to 20Mhz. However, depending on the technology used also other bandwidths maybe possible.

In a further embodiment a hysteresis function may be applied whenchanging CRS bandwidth level, thereby avoiding unnecessary switchingbetween the bandwidth modes when the cell 130 is switching from notactively serving any UEs 120 to actively serving UEs 120 very quickly.

To perform the method actions for managing transmission of CRS describedabove in relation to FIG. 3, the network node 110 may comprise thefollowing arrangement depicted in FIG. 4. As mentioned above the networknode 110 operates one or more cells and is normally configured totransmit the CRS in a first bandwidth mode.

The network node 110 comprises a radio circuitry 401 to communicate withUEs 120, a communication circuitry 402 to communicate with other networknodes and a processing unit 403.

The network node 110 is configured to, e.g. by means of an identifyingmodule 404 being configured to, identify a first cell 130 that is notactively serving any UEs 120. The network node 110 is further configuredto, or comprises a bandwidth regulating module 405 configured to, applya reduced CRS bandwidth mode of the CRS in the first cell 130 inrelation to the first bandwidth mode, when a first cell is identifiednot to actively serve any UEs 120.

The network node 110 may further be configured to, e.g. by means of abandwidth regulating module 405 further being configured to, apply thereduced CRS bandwidth mode on CRS which are sent in any subframe, exceptin a subframe where the network node 110 transmits system information,paging or random access response messages or assumes UE 120 to performmeasurements. In this embodiment the network node 110 may further beconfigured to, e.g. by means of a bandwidth regulating module 405 beingfurther configured to, send CRS over the entire bandwidth of thesubframe in the subframe in which the network node 110 transmits systeminformation, paging or random access response messages or assumes UE 120to perform measurements.

In another embodiment herein the network node 110 may further beconfigured to, e.g. by means of a bandwidth regulating module 405further being configured to, apply the reduced CRS bandwidth mode on CRSwhich are sent in any subframe, except in the first OFDM symbol of asubframe in which the network node 110 transmits system information,paging or random access response messages or assumes UE 120 to performmeasurements. In this embodiment the network node 110 may further beconfigured to, e.g. by means of an bandwidth regulating module 405 beingfurther configured to, send CRS over the entire bandwidth in the firstOFDM symbol of the subframe in which the network node 110 transmitssystem information, paging or random access response messages or assumesUE 120 to perform measurements.

The network node 110 may further be configured to, e.g. by means of abandwidth regulating module 405 further being configured to, send CRSonly in PRBs which are used for transmission in the subframe in whichthe network node 110 transmits system information, paging or randomaccess response messages or assumes UE 120 to perform measurements.

In embodiments herein, the network node 110 may further be configuredto, e.g. by means of a bandwidth regulating module 405 further beingconfigured to, send CRS only in REs which are adjacent to REs which aremapped to a common search space of the PDCCH. The common search spacecomprises the REs used by the network node 110 for sending controlinformation which is common for all UEs 120.

In order to reduce unnecessary switching between the bandwidth modes,the network node 110 may further be configured to, or may comprise thebandwidth regulating unit 405 further being configured to reduce and/orincrease the CRS bandwidth using a hysteresis function. By using ahysteresis function the network node 110 may not switch bandwidth modeimmediately when the number of connected UEs 120 changes, but willremain in one bandwidth mode for a certain amount of time after thechange of connected UEs 120 in the cell has taken place.

The embodiments herein for managing transmission of Cell ReferenceSymbols, (CRS) may be implemented through one or more processors, suchas the processing unit 403 in the network node 110 depicted in FIG. 3,together with computer program code for performing the functions andactions of the embodiments herein. The program code mentioned above mayalso be provided as a computer program product, for instance in the formof a data carrier carrying computer program code for performing theembodiments herein when being loaded into the in the network node 110.One such carrier may be in the form of a CD ROM disc. It is howeverfeasible with other data carriers such as a memory stick. The computerprogram code may furthermore be provided as pure program code on aserver and downloaded to the network node 110.

The network node 110 may further comprise a memory 406 comprising one ormore memory units. The memory 406 is arranged to be used to storeobtained information, measurements, data, configurations, schedulings,and applications to perform the methods herein when being executed inthe network node 110.

Those skilled in the art will also appreciate that the identifyingmodule 404 and bandwidth regulating module 405 described above may referto a combination of analog and digital circuits, and/or one or moreprocessors configured with software and/or firmware, e.g. stored in thememory 406, that when executed by the one or more processors such as theprocessing unit 403 as described above. One or more of these processors,as well as the other digital hardware, may be included in a singleApplication-Specific Integrated Circuitry (ASIC), or several processorsand various digital hardware may be distributed among several separatecomponents, whether individually packaged or assembled into asystem-on-a-chip (SoC).

When using the word “comprise” or “comprising” it shall be interpretedas non-limiting, i.e. meaning “consist at least of”.

The embodiments herein are not limited to the above described preferredembodiments. Various alternatives, modifications and equivalents may beused. Therefore, the above embodiments should not be taken as limitingthe scope of the invention, which is defined by the appending claims.

1. A method performed by a network node for managing transmission ofCell Reference Symbols (CRS), the method comprising: applying a firstCRS bandwidth mode with respect to a first subframe, determining that afirst cell is not actively serving any user equipments (UEs); afterdetermining that the first cell is not actively serving any UEs,applying a second CRS bandwidth mode on CRS which are transmitted in thefirst cell, wherein in the second CRS bandwidth mode the bandwidth isreduced in relation to the first CRS bandwidth mode, and the second CRSbandwidth mode is applied on CRS which are sent in any subframe, exceptin subframes in which the network node transmits system information,paging or random access response messages or assumes the UE to performmeasurements.
 2. The method of claim 1, further comprising, as a resultof determining that the first cell is not actively serving any UEs andbefore applying the second CRS bandwidth mode on CRS which aretransmitted in the first cell, determining whether or not to apply thesecond CRS bandwidth mode with respect to a particular subframe, whereindetermining whether or not to apply the second CRS bandwidth modecomprises determining: i) whether system information will be transmittedin the first cell during the particular subframe and ii) whether apaging response or random access response will be transmitted in thefirst cell during the particular subframe.
 3. The method of claim 1,wherein the network node sends CRS over the entire bandwidth of thesubframe in which the network node transmits system information, pagingor random access response messages or assumes UE to performmeasurements.
 4. The method of claim 1, wherein the second CRS bandwidthmode is applied on CRS which are sent in any subframe, except in thefirst OFDM symbol of a subframe in which the network node transmitssystem information, paging or random access response messages or assumesUE to perform measurements.
 5. The method of claim 4, wherein thenetwork node sends CRS over the entire bandwidth of the first OFDMsymbol of the subframe in which the network node transmits systeminformation, paging or random access response messages or assumes UE toperform measurements.
 6. The method of claim 1, wherein the network nodesends CRS only in Physical Resource Blocks (PRBs), which are used fortransmission in the subframe in which the network node transmits systeminformation, paging or random access response messages or assumes UE toperform measurements.
 7. The method of claim 1, wherein the network nodesends CRS only in resource elements (REs) which are adjacent to REswhich are mapped to a common search space of PDCCH.
 8. A network nodefor performing the method for managing transmission of Cell ReferenceSymbols (CRS), wherein the network node operates one or more cells andthe CRS is transmitted in a first bandwidth mode, the network node beingconfigured to: identify a first cell, which first cell is not activelyserving any UEs connected for data communication, and apply a secondbandwidth mode on CRS which are transmitted in the first cell, whereinin the second bandwidth mode the bandwidth is reduced in relation to thefirst bandwidth mode, wherein the network node further is configured to:apply the second bandwidth mode on CRS which are sent in any subframe,except in a subframe where the network node transmits systeminformation, paging or random access response messages or assumes UE toperform measurements.
 9. The network node of claim 8, wherein thenetwork node further is configured to: send CRS only in ResourceElements, REs, which are adjacent to REs which are mapped to a commonsearch space of a Physical Downlink Control Channel, PDCCH.
 10. Thenetwork node of claim 8, wherein the network node further is configuredto: send CRS over the entire bandwidth of the subframe in the subframein which the network node transmits system information, paging or randomaccess response messages or assumes UE to perform measurements.
 11. Thenetwork node of claim 8, wherein the network node further is configuredto: apply the reduced CRS bandwidth mode on CRS which are sent in anysubframe, except in the first OFDM symbol of a subframe in which thenetwork node transmits system information, paging or random accessresponse messages or assumes UE to perform measurements.
 12. The networknode of claim 11, wherein the network node further is configured to:send CRS over the entire bandwidth in the first OFDM symbol of thesubframe in which the network node transmits system information, pagingor random access response messages or assumes UE to performmeasurements.
 13. The network node of claim 8, wherein the network nodefurther is configured to: send CRS only in Physical Resource Blocks,PRBs, which are used for transmission in the subframe in which thenetwork node transmits system information, paging or random accessresponse messages or assumes UE to perform measurements.